RFC 3686 Using AES Counter Mode With IPsec ESP January 2004
encryptor can generate the IV in any manner that ensures uniqueness.
Common approaches to IV generation include incrementing a counter for
each packet and linear feedback shift registers (LFSRs).
This specification calls for the use of a nonce for additional
protection against precomputation attacks. The nonce value need not
be secret. However, the nonce MUST be unpredictable prior to the
establishment of the IPsec security association that is making use of
AES-CTR.
AES-CTR has many properties that make it an attractive encryption
algorithm for in high-speed networking. AES-CTR uses the AES block
cipher to create a stream cipher. Data is encrypted and decrypted by
XORing with the key stream produced by AES encrypting sequential
counter block values. AES-CTR is easy to implement, and AES-CTR can
be pipelined and parallelized. AES-CTR also supports key stream
precomputation.
Pipelining is possible because AES has multiple rounds (see section
2.2). A hardware implementation (and some software implementations)
can create a pipeline by unwinding the loop implied by this round
structure. For example, after a 16-octet block has been input, one
round later another 16-octet block can be input, and so on. In AES-
CTR, these inputs are the sequential counter block values used to
generate the key stream.
Multiple independent AES encrypt implementations can also be used to
improve performance. For example, one could use two AES encrypt
implementations in parallel, to process a sequence of counter block
values, doubling the effective throughput.
The sender can precompute the key stream. Since the key stream does
not depend on any data in the packet, the key stream can be
precomputed once the nonce and IV are assigned. This precomputation
can reduce packet latency. The receiver cannot perform similar
precomputation because the IV will not be known before the packet
arrives.
AES-CTR uses the only AES encrypt operation (for both encryption and
decryption), making AES-CTR implementations smaller than
implementations of many other AES modes.
When used correctly, AES-CTR provides a high level of
confidentiality. Unfortunately, AES-CTR is easy to use incorrectly.
Being a stream cipher, any reuse of the per-packet value, called the
IV, with the same nonce and key is catastrophic. An IV collision
immediately leaks information about the plaintext in both packets.
For this reason, it is inappropriate to use this mode of operation
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RFC 3686 Using AES Counter Mode With IPsec ESP January 2004
with static keys. Extraordinary measures would be needed to prevent
reuse of an IV value with the static key across power cycles. To be
safe, implementations MUST use fresh keys with AES-CTR. The Internet
Key Exchange (IKE) [IKE] protocol can be used to establish fresh
keys. IKE can also provide the nonce value.
With AES-CTR, it is trivial to use a valid ciphertext to forge other
(valid to the decryptor) ciphertexts. Thus, it is equally
catastrophic to use AES-CTR without a companion authentication
function. Implementations MUST use AES-CTR in conjunction with an
authentication function, such as HMAC-SHA-1-96 [HMAC-SHA].
To encrypt a payload with AES-CTR, the encryptor partitions the
plaintext, PT, into 128-bit blocks. The final block need not be 128
bits; it can be less.
PT = PT[1] PT[2] ... PT[n]
Each PT block is XORed with a block of the key stream to generate the
ciphertext, CT. The AES encryption of each counter block results in
128 bits of key stream. The most significant 96 bits of the counter
block are set to the nonce value, which is 32 bits, followed by the
per-packet IV value, which is 64 bits. The least significant 32 bits
of the counter block are initially set to one. This counter value is
incremented by one to generate subsequent counter blocks, each
resulting in another 128 bits of key stream. The encryption of n
plaintext blocks can be summarized as:
CTRBLK := NONCE || IV || ONE
FOR i := 1 to n-1 DO
CT[i] := PT[i] XOR AES(CTRBLK)
CTRBLK := CTRBLK + 1
END
CT[n] := PT[n] XOR TRUNC(AES(CTRBLK))
The AES() function performs AES encryption with the fresh key.
The TRUNC() function truncates the output of the AES encrypt
operation to the same length as the final plaintext block, returning
the most significant bits.
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RFC 3686 Using AES Counter Mode With IPsec ESP January 2004
Decryption is similar. The decryption of n ciphertext blocks can be
summarized as:
CTRBLK := NONCE || IV || ONE
FOR i := 1 to n-1 DO
PT[i] := CT[i] XOR AES(CTRBLK)
CTRBLK := CTRBLK + 1
END
PT[n] := CT[n] XOR TRUNC(AES(CTRBLK))
2.2. Key Size and Rounds
AES supports three key sizes: 128 bits, 192 bits, and 256 bits. The
default key size is 128 bits, and all implementations MUST support
this key size. Implementations MAY also support key sizes of 192
bits and 256 bits.
AES uses a different number of rounds for each of the defined key
sizes. When a 128-bit key is used, implementations MUST use 10
rounds. When a 192-bit key is used, implementations MUST use 12
rounds. When a 256-bit key is used, implementations MUST use 14
rounds.
2.3. Block Size
The AES has a block size of 128 bits (16 octets). As such, when
using AES-CTR, each AES encrypt operation generates 128 bits of key
stream. AES-CTR encryption is the XOR of the key stream with the
plaintext. AES-CTR decryption is the XOR of the key stream with the
ciphertext. If the generated key stream is longer than the plaintext
or ciphertext, the extra key stream bits are simply discarded. For
this reason, AES-CTR does not require the plaintext to be padded to a
multiple of the block size. However, to provide limited traffic flow
confidentiality, padding MAY be included, as specified in [ESP].
3. ESP Payload
The ESP payload is comprised of the IV followed by the ciphertext.
The payload field, as defined in [ESP], is structured as shown in
Figure 1.
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RFC 3686 Using AES Counter Mode With IPsec ESP January 2004
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initialization Vector |
| (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Encrypted Payload (variable) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Authentication Data (variable) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. ESP Payload Encrypted with AES-CTR
3.1. Initialization Vector
The AES-CTR IV field MUST be eight octets. The IV MUST be chosen by
the encryptor in a manner that ensures that the same IV value is used
only once for a given key. The encryptor can generate the IV in any
manner that ensures uniqueness. Common approaches to IV generation
include incrementing a counter for each packet and linear feedback
shift registers (LFSRs).
Including the IV in each packet ensures that the decryptor can
generate the key stream needed for decryption, even when some packets
are lost or reordered.
3.2. Encrypted Payload
The encrypted payload contains the ciphertext.
AES-CTR mode does not require plaintext padding. However, ESP does
require padding to 32-bit word-align the authentication data. The
padding, Pad Length, and the Next Header MUST be concatenated with
the plaintext before performing encryption, as described in [ESP].
3.3. Authentication Data
Since it is trivial to construct a forgery AES-CTR ciphertext from a
valid AES-CTR ciphertext, AES-CTR implementations MUST employ a non-
NULL ESP authentication method. HMAC-SHA-1-96 [HMAC-SHA] is a likely
choice.
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RFC 3686 Using AES Counter Mode With IPsec ESP January 2004
4. Counter Block Format
Each packet conveys the IV that is necessary to construct the
sequence of counter blocks used to generate the key stream necessary
to decrypt the payload. The AES counter block cipher block is 128
bits. Figure 2 shows the format of the counter block.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initialization Vector (IV) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. Counter Block Format
The components of the counter block are as follows:
Nonce
The Nonce field is 32 bits. As the name implies, the nonce is a
single use value. That is, a fresh nonce value MUST be assigned
for each security association. It MUST be assigned at the
beginning of the security association. The nonce value need not
be secret, but it MUST be unpredictable prior to the beginning of
the security association.
Initialization Vector
The IV field is 64 bits. As described in section 3.1, the IV MUST
be chosen by the encryptor in a manner that ensures that the same
IV value is used only once for a given key.
Block Counter
The block counter field is the least significant 32 bits of the
counter block. The block counter begins with the value of one,
and it is incremented to generate subsequent portions of the key
stream. The block counter is a 32-bit big-endian integer value.
Using the encryption process described in section 2.1, this
construction permits each packet to consist of up to:
(2^32)-1 blocks = 4,294,967,295 blocks
= 68,719,476,720 octets
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RFC 3686 Using AES Counter Mode With IPsec ESP January 2004
This construction can produce enough key stream for each packet
sufficient to handle any IPv6 jumbogram [JUMBO].
5. IKE Conventions
This section describes the conventions used to generate keying
material and nonces for use with AES-CTR using the Internet Key
Exchange (IKE) [IKE] protocol. The identifiers and attributes needed
to negotiate a security association which uses AES-CTR are also
defined.
5.1. Keying Material and Nonces
As described in section 2.1, implementations MUST use fresh keys with
AES-CTR. IKE can be used to establish fresh keys. This section
describes the conventions for obtaining the unpredictable nonce value
from IKE. Note that this convention provides a nonce value that is
secret as well as unpredictable.
IKE makes use of a pseudo-random function (PRF) to derive keying
material. The PRF is used iteratively to derive keying material of
arbitrary size, called KEYMAT. Keying material is extracted from the
output string without regard to boundaries.
The size of the requested KEYMAT MUST be four octets longer than is
needed for the associated AES key. The keying material is used as
follows:
AES-CTR with a 128 bit key
The KEYMAT requested for each AES-CTR key is 20 octets. The first
16 octets are the 128-bit AES key, and the remaining four octets
are used as the nonce value in the counter block.
AES-CTR with a 192 bit key
The KEYMAT requested for each AES-CTR key is 28 octets. The first
24 octets are the 192-bit AES key, and the remaining four octets
are used as the nonce value in the counter block.
AES-CTR with a 256 bit key
The KEYMAT requested for each AES-CTR key is 36 octets. The first
32 octets are the 256-bit AES key, and the remaining four octets
are used as the nonce value in the counter block.
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RFC 3686 Using AES Counter Mode With IPsec ESP January 2004
Once the attacker obtains the two plaintexts XORed together, it is
relatively straightforward to separate them. Thus, using any stream
cipher, including AES-CTR, to encrypt two plaintexts under the same
key stream leaks the plaintext.
Therefore, stream ciphers, including AES-CTR, should not be used with
static keys. It is inappropriate to use AES-CTR with static keys.
Extraordinary measures would be needed to prevent reuse of a counter
block value with the static key across power cycles. To be safe, ESP
implementations MUST use fresh keys with AES-CTR. The Internet Key
Exchange (IKE) protocol [IKE] can be used to establish fresh keys.
IKE can also be used to establish the nonce at the beginning of the
security association.
When IKE is used to establish fresh keys between two peer entities,
separate keys are established for the two traffic flows. When a
mechanism other than IKE is used to establish fresh keys, and that
mechanism establishes only a single key to encrypt packets, then
there is a high probability that the peers will select the same IV
values for some packets. Thus, to avoid counter block collisions,
ESP implementations that permit use of the same key for encrypting
outbound traffic and decrypting incoming traffic with the same peer
MUST ensure that the two peers assign different Nonce values to the
security association.
Data forgery is trivial with CTR mode. The demonstration of this
attack is similar to the key stream reuse discussion above. If a
known plaintext byte sequence P1, P2, P3 is encrypted with key stream
K1, K2, K3, then the attacker can replace the plaintext with one of
his own choosing. The ciphertext is:
(P1 XOR K1), (P2 XOR K2), (P3 XOR K3)
The attacker simply XORs a selected sequence Q1, Q2, Q3 with the
ciphertext to obtain:
(Q1 XOR (P1 XOR K1)), (Q2 XOR (P2 XOR K2)), (Q3 XOR (P3 XOR K3))
Which is the same as:
((Q1 XOR P1) XOR K1), ((Q2 XOR P2) XOR K2), ((Q3 XOR P3) XOR K3)
Decryption of the attacker-generated ciphertext will yield exactly
what the attacker intended:
(Q1 XOR P1), (Q2 XOR P2), (Q3 XOR P3)
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RFC 3686 Using AES Counter Mode With IPsec ESP January 2004
Accordingly, ESP implementations MUST use of AES-CTR in conjunction
with ESP authentication.
Additionally, since AES has a 128-bit block size, regardless of the
mode employed, the ciphertext generated by AES encryption becomes
distinguishable from random values after 2^64 blocks are encrypted
with a single key. Since ESP with Enhanced Sequence Numbers allows
for up to 2^64 packets in a single security association, there is
real potential for more than 2^64 blocks to be encrypted with one
key. Therefore, implementations SHOULD generate a fresh key before
2^64 blocks are encrypted with the same key. Note that ESP with 32-
bit Sequence Numbers will not exceed 2^64 blocks even if all of the
packets are maximum-length IPv6 jumbograms [JUMBO].
There are fairly generic precomputation attacks against all block
cipher modes that allow a meet-in-the-middle attack against the key.
These attacks require the creation and searching of huge tables of
ciphertext associated with known plaintext and known keys. Assuming
that the memory and processor resources are available for a
precomputation attack, then the theoretical strength of AES-CTR (and
any other block cipher mode) is limited to 2^(n/2) bits, where n is
the number of bits in the key. The use of long keys is the best
countermeasure to precomputation attacks. Therefore, implementations
that employ 128-bit AES keys should take precautions to make the
precomputation attacks more difficult. The unpredictable nonce value
in the counter block significantly increases the size of the table
that the attacker must compute to mount a successful attack.
8. Design Rationale
In the development of this specification, the use of the ESP sequence
number field instead of an explicit IV field was considered. This
selection is not a cryptographic security issue, as either approach
will prevent counter block collisions.
In a very conservative model of encryption security, at most 2^64
blocks ought to be encrypted with AES-CTR under a single key. Under
this constraint, no more than 64 bits are needed to identify each
packet within a security association. Since the ESP extended
sequence number is 64 bits, it is an obvious candidate for use as an
implicit IV. This would dictate a single method for the assignment
of per-packet value in the counter block. The use of an explicit IV
does not dictate such a method, which is desirable for several
reasons.
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RFC 3686 Using AES Counter Mode With IPsec ESP January 2004
1. Only the encryptor can ensure that the value is not used for more
than one packet, so there is no advantage to selecting a mechanism
that allows the decryptor to determine whether counter block
values collide. Damage from the collision is done, whether the
decryptor detects it or not.
2. Allows adders, LFSRs, and any other technique that meets the time
budget of the encryptor, so long as the technique results in a
unique value for each packet. Adders are simple and
straightforward to implement, but due to carries, they do not
execute in constant time. LFSRs offer an alternative that
executes in constant time.
3. Complexity is in control of the implementer. Further, the
decision made by the implementer of the encryptor does not make
the decryptor more (or less) complex.
4. When the encryptor has more than one cryptographic hardware
device, an IV prefix can be assigned to each device, ensuring that
collisions will not occur. Yet, since the decryptor does not need
to examine IV structure, the decryptor is unaffected by the IV
structure selected by the encryptor. One cannot make use of the
same technique with the ESP sequence numbers, because the
semantics for them require sequential value generation.
5. Assurance boundaries are very important to implementations that
will be evaluated against the FIPS Pub 140-1 or FIPS Pub 140-2
[SECRQMTS]. The assignment of the per-packet counter block value
needs to be inside the assurance boundary. Some implementations
assign the sequence number inside the assurance boundary, but
others do not. A sequence number collision does not have the dire
consequences, but, as described in section 6, a collision in
counter block values has disastrous consequences.
6. Coupling with the sequence number is possible in those
architectures where the sequence number assignment is performed
within the assurance boundary. In this situation, the sequence
number and the IV field will contain the same value.
7. Decoupling from the sequence number is possible in those
architectures where the sequence number assignment is performed
outside the assurance boundary.
The use of an explicit IV field directly follows from the decoupling
of the sequence number and the per-packet counter block value. The
overhead associated with 64 bits for the IV field is acceptable.
This overhead is significantly less than the overhead associated with
Cipher Block Chaining (CBC) mode. As normally employed, CBC requires
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RFC 3686 Using AES Counter Mode With IPsec ESP January 2004
a full block for the IV and, on average, half of a block for padding.
AES-CTR with an explicit IV has about one-third of the overhead as
AES-CBC, and the overhead is constant for each packet.
The inclusion of the nonce provides a weak countermeasure against
precomputation attacks. For this countermeasure to be effective, the
attacker must not be able to predict the value of the nonce well in
advance of security association establishment. The use of long keys
provides a strong countermeasure to precomputation attacks, and AES
offers key sizes that thwart these attacks for many decades to come.
A 28-bit block counter value is sufficient for the generation of a
key stream to encrypt the largest possible IPv6 jumbogram [JUMBO];
however, a 32-bit field is used. This size is convenient for both
hardware and software implementations.
9. IANA Considerations
IANA has assigned 13 as the ESP transform number for AES-CTR with an
explicit IV.
10. Intellectual Property Statement
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intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
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IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
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obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
11. Acknowledgements
This document is the result of extensive discussions and compromises.
While not all of the participants are completely satisfied with the
outcome, the document is better for their contributions.
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RFC 3686 Using AES Counter Mode With IPsec ESP January 2004
14. Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
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Housley Standards Track [Page 19]